Cavity Wall materials & Cost calculator.
This Calculator will estimate the quantities of bricks, blocks and mortar (sand & cement) insulation, ties & render
required for a given area of cavity wall
construction.
Please enter the dimensions in the white fields below and click calculate to display the results.
See below for advice on cavity wall construction.
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The costs shown can only ever be used as a very rough guide, materials and labour
costs vary greatly by geography, season, workload and many
other factors.
Also the blockwork and brickwork prices shown are based on a certain amount of assumption
and averaging on our part.
Good Practice
- Brick Mixing - Natural materials such as clay bricks have
characteristic variations in shade, colour and texture. However because
patching or colour banding would not be desired in the finished work, it is
important to blend the bricks.
Although
this is carried out at manufacturing there is still a need to further carry
out this process on site, this is best achieved mixing from as many packs as
possible and mixing from corners rather than layers to maximise colour
consistency.
- Mortar - The correct preparation is key to the overall aesthetics,
durability, weather resistance and structural performance.
- Mix Design - mortar is a combination of binder (cement or lime) with sand.
Either with or without fillers or additives such as plasticiser. Good mortar
design fills the voids in a well graded sand with the binder or filler. This
is what produces the ratio.
In general it is good practice to
use the weakest possible mortar mix that is compatible with the brick or
block being used and the exposure, unless otherwise specified. This mortar
mix will exhibit less shrinkage and produce a superior brick/mortar bond,
weaker mixes also tend to be more flexible.
- Aesthetics - The colour and joint profile chosen will affect the overall
appearance of the brickwork.
Whether the aim is to contrast
against the brickwork or blend subtly there is a wide choice.
- Durability - Water and frost are the main elements that will affect
brickwork. However, as the frost resistance of the mature mortar increases
with the proportion of cement, designation (i) and (ii) mortars are
considered more suitable for severely exposed applications such as in
copings, cappings and cills and below DPC level. Designation (i) mortars
should be used for paving.
If there are high levels of soluble
salts and/or the brickwork is likely to be saturated for periods then the
Ordinary Portland Cement should be replaced with sulphate-resisting Portland
cement.
- Rain - Resistance to this can be achieved or increased by good
adhesion, well filled joints and an appropriate joint profile.
Tooled bucket handle or weather
struck joints are the most appropriate for exposed areas. Whilst recessed
joints are only suitable for moderate exposure.
- Structural - In design calculations, it is assumed that both the
compressive strength value of the appropriate mortar designation is achieved
in practice and that the joints are completely filled with mortar. This
underlines the importance of good building practice.
- Adhesion - Strength of bond is affected by the suction rate of the
brick, the mortar constituents, water content and the weather.
Where the suction rate of a brick
exceeds the optimum value of 1.5kg/m2/min, it is recommended that the bricks
are wetted prior to laying, particularly in hot & dry weather conditions.
In general
terms the correct proportions of binders and sand is necessary for consistent
mortar strength, avoidance of colour variation and a good bond between brick
& mortar. This should be achieved by the use of batching boxes
buckets, avoid the use of shovels for proportioning by volume.
- Bricklaying
- Mortar Consistency: In general, the correct mortar consistency
is a matter of the bricklayers judgement. However, if mortars are too wet at
the time of laying this can lead to staining of the brick edges and, in the
worst cases, runs of mortar can form on the face of the brick. Ideally,
mortar should 'hang' on the trowel without being sticky, should spread easily
and should not ooze out of the joints when bricks are positioned.
- Docking: It is accepted practice for bricks with a high absorbency to
be wetted prior to laying. This procedure adjusts the suction rate of the
brick to promote good adhesion between the brick and the mortar. However,
bricks should not be soaked during the process as this could lead to
subsequent staining problems. See above. Bricks with a low suction rate
should never be wetted because they might 'float'; mortar that is mixed too
wet will have the same effect.
- Tooling: The bricklayer uses his skill and experience in determining
when to begin tooling the joints after placing the bricks in mortar. Early
tooling should be avoided as this can produce a dragged effect on the mortar
surface and the cement fines may be drawn to the surface resulting in an
unsightly patchy bloom on the joints in the mature work.
- Winter Working: If the temperature of unhardened mortar is less
than 40C,
the performance of the joints produced will be inadequate. Consequently,
bricklaying should stop when the air temperature
reaches 30C and falling, unless
the mortar temperature can be maintained at a minimum
of 40C until it has
hardened. Bricklaying can be resumed when the air temperature
rises to 10C and is expected to
continue rising to above 30C over the bricklaying period.
Special Precautions in Cavity Wall Construction
- Wherever possible, contact between 2 leaves should be avoided
- The cavity should be kept clear of
droppings, and any on ties should be removed
- The main horizontal damp
proof course must be in two separate widths, and the bottom of the cavity
must be at least 150mm below this
- Heads of openings should be protected by a suitable cavity tray. Jambs must not be
solid unless insulated damp proof courses are provided
- Projecting ends of stone
or terra-cotta sills should be notched back from the inner leaf
- Cover flashings at
intersections of lower buildings adjoining cavity walls must extend to at
least 25mm beyond the inner face of the outer leaf.
- Weep holes should be
formed at the main horizontal damp proof course and above the damp proof
courses over openings. No other ventilation to
the cavity should be provided.
- Built in floor timbers
should be sound and their ends treated with preservative/capped
- The cavity should be
continued up a parapet wall to the coping
- Ties should be rust
proof, capable of preventing rain transmission and easily cleaned of
droppings.